There are many quotes (in Mary Shelley's novel Frankenstein) which refer to the dangerousness or forbidden nature of Victor's pursuit of knowledge.

The first quote appears in Chapter Two. Here, Victor is admitting that the knowledge he seeks ("the secrets of heaven and earth") are ones which should not necessarily be looked for. By simply using the word "secret" Victor is admitting that the knowledge he is seeking is forbidden.

It was the secrets of heaven and earth that I desired to learn; and whether it was the outward substance of things, or the inner spirit of nature and the mysterious soul of man that occupied me, still my inquiries were directed to the metaphysical, or, in it highest sense, the physical secrets of the world.

Later in Chapter Two, Victor again speaks about the secrets of nature and his desire to uncover them. (As stated before, Victor's quest to dissolve the "secrets" stands as dealing with the forbidden knowledge.)

I have described myself as always having been embued with a fervent longing to penetrate the secrets of nature.

One last quote from Chapter Two offers another example of why the knowledge Victor was seeking could be considered forbidden or dangerous.

The raising of ghosts or devils was a promise liberally accorded by my favourite authors, the fulfillment of which I most eagerly sought; and if my incantations were always unsuccessful, I attributed the failure rather to my own inexperience and mistake than to a want of skill or fidelity in my instructors.

Once again, the reference of dealing with "things" which should be left alone ("raising of ghosts or devils") would normally make one consider what they were doing and the kind of knowledge they were seeking. Unfortunately for Victor, his desires become obsessive and he fails to see any type of quest for knowledge as forbidden or dangerous.

One last and final quote that speaks to the forbidden and dangerous pursuit of knowledge is found in Chapter Three. Victor, at school in Ingolstadt, finds himself desiring the greatest of all knowledge. He, unhappy with what has already been discovered, wishes to (basically) find out all of the scientific mysteries on earth.

So much has been done, exclaimed the soul of Frankenstein—more, far more, will I achieve: treading in the steps already marked, I will pioneer a new way, explore unknown powers, and unfold to the world the deepest mysteries of creation.

Basically, Victor does not recognize that some secrets are meant to stay buried.